1. Field of the Invention
The present invention relates to a device for positioning a tool within the flow string of a wellbore and more particularly of a petroleum-producing wellbore.
2. Description of the Prior Art
Branched wells or so-called multiple-completion wells in which lateral boreholes are connected to a main vertical wellbore and are inclined at a predetermined angle with respect to the main wellbore are coming into increasingly widespread use, especially in order to increase the production of the main well, also known as the "parent" well.
The processes of drilling of branched multiple-completion wells and recovery from producing formations both in the case of petroleum reservoir exploitation and in the case of subsurface mining are practiced for many reasons which include economic considerations or factors relating to the nature of the productive stratum or mineral deposit.
In fact, the lateral branches of a parent well not only permit an increase in production but also permit maximum recovery of the contents of a producing formation. In addition, these branch-type boreholes permit extension of the subsurface producing zone by means of a single surface installation and also make it possible to reduce the high flow resistance which appears in the vicinity of the borehole within a parent well.
A further practical advantage of branch-type boreholes lies in the possibility of recovering oil and gas from formations or strata which have a large number of fractures. This possibility arises from the fact that the lateral branch lines intercept the fractures and connect them to the drainage system of the parent well.
Finally, branched multiple-completion wells are widely utilized for effective exploitation of a pay formation in which the top or superincumbent stratum of the reservoir has a highly irregular relief and/or a small thickness.
In the prior art, branch-type boreholes were drilled in a direction away from the parent well with a deflecting tool known as a whipstock. It is readily apparent that branch-boreholes are smaller in diameter than the parent wellbore and the angle of deflection of each branch-borehole is chosen as a function of the producing formation zone into which the branch hole is intended to open. In order to minimize drilling costs, the same whipstock or deflecting tool is sometimes employed for drilling two branch holes which are symmetrical with respect to the parent wellbore by rotating the deflecting tool through an angle of 180.degree. within the parent wellbore. Branched multiple-completion wells drilled in this manner by means of deflecting tools are described in U.S. Pat. Nos. 4,396,075 and 4,415,205.
When each branch-borehole has been completely drilled, the elements required for recovery of oil or gas from the deposit are then run into the hole. When all the branch holes are ready for use, all equipment which serves no useful purpose for production are withdrawn from the parent well. One item of such equipment is the whipstock, for example.
A point worthy of note at this juncture is that the structure of a parent well associated with branch-type boreholes in a multiple-completion oil-well installation is attended by a disadvantage in that no provision is made for any selective connection between the bottom of each branch hole and the surface.
Throughout the foregoing discussion, reference has been made to lateral branch lines or boreholes. It will be understood, however, that the branch-boreholes can extend obliquely, horizontally or in any direction provided that they are associated with a main or parent wellbore to which they are joined in convergent relation.
As long as the branch lines are in service or in other words are operating under production conditions of continuous flow, no major difficulty arises.
In contrast, when one or a number of branch lines has been put out of service and it is desired to reactivate or put the line back into service, major difficulties are then encountered.
The first of these difficulties is primarily related to the fact that, during the period of service, the branch line (tubing string) has become clogged with drill cuttings and debris which it is very difficult to locate with any degree of accuracy. It would admittedly be possible each time a whipstock is run into a wellbore for drilling a branch hole to note very precisely its position within the parent wellbore and then to replace the deflecting tool at exactly the same location. Such an operation, however, appears to be very difficult to perform in practice.
The second difficulty lies in the fact that, even assuming that it were possible to relocate the original branch hole, it would be absolutely essential to reuse the same deflecting tool either for removing all debris from this reactivated branch hole or to carry out a second drilling operation.
In view of these major difficulties, it has thus far proved preferable not to put old branch holes back into service but to drill new holes. Even though it is true that drilling of a branch hole is not comparable with the work involved in drilling a main vertical wellbore, it still remains a very costly operation.
In order to overcome the drawbacks discussed in the foregoing, it has already been proposed to provide a device which permits normal production when branch lines are in service but which also makes it possible beforehand to disconnect a lost casing joint (length of casing) which may subsequently be reconnected at the surface, if necessary with a view to reactivating one or a number of branch lines after they have been put out of service, this being possible without entailing high capital expenditure.
A device of this type is described in French patent application No. 83 13981. The device for drilling and initiating production of an oil or gas well formed by a parent wellbore and at least one branch-borehole which opens into said parent wellbore includes an outer casing placed within the parent wellbore, at least one branch assembly fixed in situ within said casing and provided with at least one fixed branch tube whose lower end communicates with a branched multiple-completion well. By means of a connecting assembly associated with a positioning assembly, any branch hole can be drilled and put back into service after use, this being made possible by a fixed positioning-assembly spline on which a cam profile of the connecting assembly is applied.
In practice, however, it has been found that, each time it proves necessary to carry out a servicing operation within a lateral branch hole or branch tubing string during the production stage, it is necessary to raise the pump and production tubing to the surface, then to lower the drill pipe. When the servicing operation is completed, the production tubing and pump are replaced in position. These operations involving raising and lowering of production tubing and pump are not only costly in themselves but above all result in outage of the entire oil-well installation over relatively long periods of time. From a production standpoint, however, it is clearly advisable for both technical and economic reasons to avoid such outages wherever possible.
The present invention solves the problem set forth in the foregoing by considerably reducing the outage time of a producing well. To this end, a device is proposed for correctly positioning a servicing tool within the borehole concerned without having to raise the production tubing to the surface.
It should be understood that the term "servicing operation" used in the foregoing and throughout this specification is intended to mean any operation to be performed within a lateral borehole, designated hereinafter as a branch flow string. For example, operations of this type can include interference measurements or else water-exclusion plug-back operations.